1. Field of the Invention
An optically amplifying device with a gain equalizing function of the present invention relates to an optically amplifying device including an optical amplifier for optically amplifying directly an input signal light beam. In particular, the present invention relates to an optically amplifying device for optically amplifying a WDM signal (a Wavelength Division Multiplexed signal) which is obtained by wavelength-division multiplexing signal light beams having a plurality of wavelengths different from one another.
2. Description of the Related Art
In a middle and long distance optical fiber communication system, there is employed an optical amplifier for amplifying a signal light beam which has been attenuated through a transmission line fiber. As the optical amplifier, there is widely used an optical fiber amplifier for amplifying directly a signal light beam with a rare-earth doped optical fiber as an amplification medium. Also, as the amplification medium of the optical fiber amplifier, the rare-earth doped optical fiber is employed. In this connection, there is generally known an FDFA (an Erbium Doped Fiber Amplifier) employing erbium elements as dopant.
On the other hand, as a system for increasing the transmission capacity without establishing more optical transmission lines, there is generally known a wavelength division multiple transmission system for wavelength-division multiplexing a plurality of signal light beams having wavelengths different from one another to optically transmit the resultant optical signal. In this wavelength division multiple optical transmission system as well, the above-mentioned optical fiber amplifier can be applied thereto.
However, if the constituent signal light beams of the wavelength division multiplexed signal light beam are intended to be optically amplified by the optical fiber amplifier, the dispersion between the gains for the constituent signal light beams due to the wavelength characteristics of the gain of the erbium doped optical fiber, i.e., the degradation of the flatness of the gains becomes a problem. Then, for the purpose of improving the gain flatness, a gain equalizer is employed.
For the conventional gain equalizer, there is employed an optical passive component having the wavelength characteristics of the loss which are set so as to cancel the waveform characteristics of the gain of the erbium doped optical fiber. This aim is such that in the case where the gain flatness can not be ensured due to the fact that as compared with the gain of a signal light beam having a certain wavelength, the gain of a signal light beam having another wavelength is lower, an optical passive component which is larger in loss than the former signal light beam is inserted and disposed in order to adjust the total gain.
However, if the gain flatness is intended to be ensured on the basis of such a construction, for example, in the case where the optical amplifier has the wavelength characteristics of the gain of 5 dB between the signal light beams of the wavelength division multiplexed signal light beam, the gain difference can not be perfectly cancelled out unless the loss of at least 5 dB is added thereto. Therefore, the loss required for the equalization becomes large, and hence finally, the optical output characteristics required for the overall optically amplifying device can not be obtained. In addition, there arises the problem that the noise degradation is increased.